The present invention relates to optical preform fabrication in general, and more particularly to an arrangement for removing glass soot tending to accumulate in the interior of an exhaust tube during a chemical vapor deposition phase of the preform fabrication process.
There is already known and in use a technique of fabricating optical fiber preforms which involves a phase that is sometimes called a modified chemical vapor deposition process. During this fabrication phase, which will be hereafter occasionally referred to as chemical vapor deposition phase, a gaseous medium containing certain glass precursors, especially halides or other compounds of glass forming or doping elements, such as silicon, boron, germanium, phosphorus or the like, as well as on oxidizing agent, such as oxygen, is caused to flow through the interior of a substrate tube which is usually rotated about its longiutudinal axis and is heated from the outside, for instance by an oxyhydrogen flame applied to the external surface of the substrate tube, so that chemical reactions take place at and/or downstream of the heated region of the substrate tube with attendant conversion of the glass precursors into glass materials which become deposited in the form of at least one layer at the internal surface of the substrate tube.
During this phase, glass soot that is constituted by minute particles of the glass materials is formed at or downstream of the heated region and is then carried by the gaseous medium out of the downstream end of the substrate tube and into what is called an exhaust tube. This exhaust tube is a glass tube with an enlarged diameter with respect to the substrate tube and fused to the downstream end of the substrate tube at a merger or joinder region, so that it shares in the rotational movement of the substrate tube. Experience has shown that the soot which reaches the interior of the exhaust tube tends to deposit therein and form accumulations which, in effect, reduce the flow-through cross-sectional area of the exhaust tube. Such accumulations could then interfere with the performance of the chemical vapor deposition process and ultimately with the properties of the layer or layers deposited in the substrate tube by changing the deposition parameters, such as pressure, flow speed of the gaseous medium or the like, from those originally selected.
The method that is currently most commonly used to avoid this problem by removing the soot from the exhaust tube before the soot accumulations can cause any significant change in the deposition parameters is for the operator of the lathe on which the preform is being fabricated to periodically scrape the interior of the exhaust tube using a quartz rod which is approximately one meter long to reach all the way into the exhaust tube. Once loosened, the soot is then drawn from the exhuast tube with the same quartz rod using a series of raking motions. It was established that this soot removal technique suffers of several disadvantages. One of such drawbacks of this techqnique is that it requires frequent operator intervention in an otherwise automated process. Also, the use of this technique can subject the preform being fabricated to severe contamination, since some of the loosened soot can be returned into the interior of the substrate and become a part of the layer being formed therein.